1. Field of the Invention
The present invention relates to a capacitive touch panel and an electrode set thereof, and more particularly to a capacitive touch panel having symmetrical capacitive and resistive values, satisfactory signal-to-noise ratio (SNR) and interference withstand capability.
2. Description of the Related Art
With reference to FIG. 7, a first conventional one-dimensional single layer capacitive touch panel has a substrate 50, an Indium Tin Oxide (ITO) electrode set and a flexible control board 60. The ITO electrode set is formed on the substrate 50 and has multiple electrode couples 51. Each electrode couple 51 has a first electrode 511 and a second electrode 512. The flexible control board 60 is connected to one side of the substrate 50 and has a controller 61 formed thereon. The controller 61 has multiple signal terminals 611 electrically connected to the respective first electrodes 511 and the second electrodes 512 of the electrode couples 51 for scanning and locating an object 70 on the substrate 50.
As for a method performed by the flexible control board 60 to detect object 70 on the substrate, a U.S. Pat. No. 8,121,283, entitled “Tapered capacitive sensing structure”, discloses that each of the first electrode 511 and the second electrode 512 of one of the electrode couples 51 covered by the object 70 generates a capacitance value with respect to the ground when the object 70 touches the substrate 50. As areas of the first electrode 511 and the second electrode 512 corresponding to the object 70 are different, the capacitance values generated by the first electrode 511 and the second electrode 512 also differ from each other. For example, when the electrode couple 51 touched by the object 70 at a coordinate Y2 in a Y axis generates a capacitive value, the coordinate in the Y axis of the object 70 can be determined as Y2. Furthermore, as the capacitance value of the second electrode 512 is greater than that of the first electrode 511 and a ratio between the sensing values of the first electrode 511 and the second electrode 512 can reflect an X coordinate of the object 70 on the electrode couple 51, the X coordinate of the object 70 can be determined as X2. The two-dimensional coordinates (Y2, X2) of the object 70 can thus be obtained.
With reference to FIG. 8, a second conventional one-dimensional single layer capacitive touch panel differs from the foregoing conventional capacitive touch panel in higher sensing accuracy. The first electrode 511 of each electrode couple 51 has two tapered traces 511a, 511b. The second electrode 512 of the electrode couple 51 has two tapered traces 512a, 512b. Wide ends of the two tapered traces 511a, 511b of the first electrode 511 are connected, and a signal terminal 513 is formed on a connected portion of the wide ends of the two tapered traces 511a, 511b of the first electrode 511. Wide ends of the two tapered traces 512a, 512b of the second electrode 512 are connected, and another signal terminal 514 is formed on a narrow end of one of two tapered traces 512b of the second electrode 512. The signal terminals 513, 514 are respectively connected to the signal terminals 611 of the controller 61 on the flexible control board 60.
As the first electrode 511 and the second electrode 512 of each electrode couple 51 have more conductive traces, the sensing accuracy of the one-dimensional single layer capacitive touch panel is enhanced. Currently, each of the first electrode 511 and the second electrode 512 can be developed to provide up to three tapered traces. With reference to FIG. 9, each of the first electrode 511 and the second electrode 512 of each electrode couple 51 has three tapered traces 511a˜511c, 512a˜512c. Wide ends of the tapered traces 511a˜511c of the first electrode are connected, and a signal terminal 513 is formed on a connected portion of the wide ends of the three tapered traces 511a˜511c of the first electrode 511. Wide ends of the three tapered traces 512a˜512c of the second electrode 512 are connected, and another signal terminal 514 is formed on a narrow end of outmost one 512c of the three tapered traces of the second electrode 512.
Although enabling one-dimensional single layer touch panel to have enhanced sensing accuracy, the foregoing electrode structures are noise-prone in determining coordinates of objects on the touch panel. Because the tapered traces 511a˜511c, 512a˜512c of the first electrode 511 and the second electrode 512 are formed of Indium Tin Oxide (ITO) and the material of ITO has an internal resistance, a length of signal transmission path in the tapered traces 511a˜511c, 512a˜512c directly affects sensed capacitance and resistance thereof With reference to FIG. 10, given two locations of objects 70A, 70B marked on the electrode couple 51 as an example, a path L1 from the signal terminal 513 to one of the objects 70A is shorter than another path L2 from the signal terminal 514 to the other object 70B. Under the circumstance, the resistance of the path L1 (measured at approximately 20.5Ω) almost doubles the resistance of the path L2 (measured at approximately 10.7Ω). In other words, the capacitance and resistance of the first electrode 511 and the second electrode 512 are asymmetrical. Thus, location determination of touch object is error-prone due to noise when the first electrode 511 and the second electrode 512 are charged or discharged.
With reference to FIG. 11A, a period for signal terminals to output scan signals includes a charge time TCH and a sense time TS. The charge time TCH of the first electrode 511 and the second electrode 512 is defined between when the first electrode 511 and the second electrode 512 are initially charged and fully charged. Sensed capacitance values S1, S2 of the first electrode 511 and the second electrode 512 are read during the sense time TS right after the charge time TCH expires. Hence, as long as the charge time TCH is long enough, despite the differences in capacitance and resistance between the first electrode 511 and the second electrode 512, the sensed capacitance can still be read successfully when the first electrode 511 and the second electrode 512 are fully charged. However, with reference to FIG. 11B, if the touch panel is subjected to noise interference during the charging processes of the first electrode 511 and the second electrode 512, the differences in capacitance and resistance will affect an actual full charge time of the first electrode 511 and the second electrode 512. As a result of the differences in capacitance and resistance, the sensed capacitance values S1, S2 read during the sense time TS are not proportional to each other and false identification of coordinates of objects on the touch panel becomes inevitable. Since the most direct noise source of touch panels comes from interfered scan signals of a liquid crystal display (LCD) touch panel, the possibility for the foregoing false identification to take place is rather high. Effective solution should be addressed to tackle the asymmetrical capacitance and resistance of the first electrode and the second electrode.